Antibodies are one of our most effective anti-pathogen defenses and rapid production of antibodies is critical in most immune responses. At the same time, autoantibodies are pathogenic mediators in various autoimmune diseases. The mechanisms ensuring that we respond efficiently to foreign antigens but make minimal antibody responses to self-antigens are only partially understood. Long-term goals of this proposal are to define the cues controlling positioning of B cells before and during the immune response and to understand the factors that promote elimination of autoantigen binding B cells in peripheral tissues. Combined, these studies explore links that may exist between microenvironmental positioning and access to prosurvival factors. Several approacheswill be taken to study these issues. One approach will be to characterize of the mechanism by which the B cell trophic factor, BAFF, promotes B cell survival. The possibility that BAFF functions to protect B cells from BIM mediated elimination will be tested. BAFF is a strong regulator of the transcription factor, NFkB p52, and the impact of BAFF on gene transcription in mature B cells will be studied. A second related aim will explore a preliminary finding that naive T cells provide a factor that augments autoreactive B cell survival. CD40L will be tested as a candidate for this factor and the mechanisms regulating CD40L availability will be explored. Chemokines have been found to play a key role in controlling B cell positioning and recent findings also implicate adhesion molecules and the lysophospholipid, sphingosine-1-phosphate (S1P), in the regulation of some B cell positioning events. The mechanism by which S1P receptor-1 promotes B cell positioning in the splenic marginal zone will be explored. The mechanisms determining whether plasma cells remain in their organ of generation or migrate to the bone marrow will also be characterized. Finally, the requirements for B cell positioning in the light and dark zones of the germinal center will be further defined and the dynamics of cell movement within the germinal center will be studied using multiphoton microscopy. As well as improving our understanding of factors regulating the efficiency of B cell antibody production to foreign pathogens, these studies are likely to provide insight into how defects in B cell trafficking and homeostasis contribute to autoimmunity or mmunodeficiency.